1. Field of the Invention
The present invention relates to a piezoelectric vibrator having good high frequency characteristics and a manufacturing method thereof.
2. Description of the Prior Art
Recently, communication devices have been made more compact and to operate at higher frequencies due to the development of mobile communication technology. These devices need vibrators and high frequency filters, and piezoelectric vibrators can be used for such vibrators and high frequency filters.
A prior art piezoelectric vibrator such as a quartz vibrator is prepared by forming a quartz plate of a prescribed shape and by forming electrodes on the top and bottom planes thereof. Vibrations are generated in the piezoelectric vibrator by applying an alternating electric voltage between the electrodes. A quartz vibrator having good characteristics at high frequencies is needed for mobile communication devices. Among high frequency characteristics, vibration frequency and Q of resonance (which is in correspondence to an inverse of loss) are important. The vibration frequency of a quartz vibrator is inversely proportional to the thickness of the quartz plate if thickness-shear vibration of quartz is used. Usually, a quartz vibrator which can be produced easily has a vibration frequency of 50 MHz or less in the fundamental mode and has a quartze plate with a thickness of 30 .mu.m or more. In order to increase the vibration frequency, it is needed to reduce the thickness of the quartz plate. For example, the vibration frequency of 167 MHz can be available for a thickness of 10 .mu.m, and the vibration frequency of 330 MHz can be available for a thickness of 5 .mu.m. For example, E. A. Gerber et al. produces quartz vibrators for frequencies of hundreds of MHz to more than 1 GHz by using polishing, etching, ion trimming or the like ("Advances in Microwave Acoustic Frequency Sources", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 10, October 1986, 1004). However, the Q of resonance decreases with increasing frequency. This is ascribed partly to the increase in electrode loss due to the skin effect at the electrodes. This is also ascribed to the weight of the electrode itself which contributes more loss with decreasing thickness. Then, there is an upper limit on the thickness of the electrode though the increase in thickness decreases the resistance of the electrode. Therefore, if quartz is formed to be thin with polishing or the like, the vibration frequency increases whereas the Q of resonance decreases, or a piezoelectric vibrator of good high frequency characteristics cannot be produced.
On the other hand, a quartz vibrator with a non-contact electrode is also known. For example, a quartz vibrator disclosed in Japanese Patent laid open Publication 2-121512/1990 intends to decrease temperature hysteresis. A quartz plate having a concave section is interposed and fixed between top and bottom plates on which an electrode is formed. Gaps are provided between the quartz plate and the electrodes. A specified metal (SUS 630) is used for metallic plates for holding the quartz plate, and the quartz plate is fixed with the metallic plates or holders with screws at a fixing angle of the quartz plate determined so that the thermal expansion coefficient of the metallic plates agrees with that of the quartz plate at the fixing angle. The thermal hysteresis decreases because the thermal expansion coefficient of the metallic plates is chosen to agree with that of the quartz plate. In the structure of the quartz vibrator, the effect of the mass of the electrodes on the vibration can be reduced so that Q at high frequencies can be improved. However, because screws are used, it is difficult to reproducibly provide a constant gap on the order of a micrometer order. If the gap cannot be adjusted at a high precision for a quartz plate of thickness of 10 .mu.m or less, the stability of Q on production and on the characteristics under operation are not sufficient. Further, because screws are used, it is difficult to seal the vibration element in a compact size. The thermal expansion coefficient cannot agree only at a certain direction, and it is difficult to realize stability in a wide temperature range. The characteristics are liable to change if dropped or due to mechanical vibrations. It is also known to use an adhesive instead of screws to provide gaps and to fix the holders. However, an adhesive is an organic material and has a low heat-resisting property, and the characteristics may change for soldering at 220.degree. C. or more and the temperature characteristics on use or on transport are not stable. Further, if the sealing is performed, gas may be generated from the adhesive and this may change the characteristics. A similar quartz vibrator is also disclosed in Japanese Patent laid open Publication 5-22071/1993, wherein a quartz plate and a pair of plates each having a non-contact electrode are fixed mechanically with screws.
Further, an elastic surface wave element disclosed in Japanese Patent laid open Publication 3-6912/1991 has a non-contact type electrode with a gap between the piezoelectric plate and the electrode. This element also uses the non-contact type electrodes like the present invention. However, the structure of the electrode is different from that of the present invention because an elastic surface wave is generated. Further, a bridge or supporter of the electrode is formed by using deposition on the piezoelectric plate, while the supporter of the electrode is bonded directly with the piezoelectric plate in the present invention.